EFFECTS OF SILL BATHYMERY, OSCILLATING BAROTROPIC FORCING AND VERTICAL MIXING ON ESTUARY OCEAN EXCHANGE

Intratidal and low-pass filtered fluctuations of density stratificatio n in eastern Long Island Sound are apparently produced by both hydraul ic effects and tidally induced vertical mixing. In order to evaluate t he effects of bathymetry, oscillating barotropic forcing and vertical mixing on density stratification and on the exchange between estuarine and oceanic waters, a series of numerical experiments over a simplifi ed sill bathymetry are performed. Hydraulic effects over a sill decrea se exchange by 25% relative to flat bottom. There is a fundamental cha nge in the nature of the flow from weak to strong barotropic forcing: during weak tidal forcing, sill exchange can be subject to hydraulic c ontrol and the intratidal stratification is determined by internal wav e motion on the pycnocline; during strong forcing, the density field i s determined by the gradient advection of the predominant barotropic t ide, the hydraulic control is broken, and exchange is determined by th e tidal prism. Vertical mixing tends to decrease exchange and to break the hydraulic control.